You don't mention the mail server platform you are using, so I'm guessing you are looking for an additional server to process mail before it gets delivered to your primary mail server.
You could set up a Linux box running Sendmail for your MTA, and MailScanner with SpamAssassin for spam filtering. You can also incorporate virus scanning with MailScanner. It will integrate with most antivirus solutions for Linux.
This setup will integrate with pretty much any mail setup you have. This is what I use. I am not an ISP by any means, but I host mail for several domains and it works very well. The virus scanning is awesome, and the spam filtering is great.
Go to www.mailscanner.info for information on Mailscanner.
Check out my mail scanner statistics page for info on my mail server:
www.codycrew.net/mrtg2
Email me if you want to try this but don't really know how to get started with Linux.